Thermal management of printed circuit board components

ABSTRACT

A first thermal management approach involves an air flow through cooling mechanism with multiple airflow channels for dissipating heat generated in a PCA. The air flow direction through at least one of the channels is different from the air flow direction through at least another of the channels. Alternatively or additionally, the airflow inlet of at least one channel is off-axis with respect to the airflow outlet. A second thermal management approach involves the fabrication of a PCB with enhanced durability by mitigating via cracking or PTH fatigue. At least one PCB layer is composed of a base material formed from a 3D woven fiberglass fabric, and conductive material deposited onto the base material surface. A conductive PTH extends through the base material of multiple PCB layers, where the CTE of the base material along the z-axis direction substantially matches the CTE of the conductive material along the x-axis direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 15/639,788filed on Jun. 30, 2017 as a national phase application of PCTInternational Application No. PCT/IL2015/051093 with an InternationalFiling Date of Nov. 11, 2015, which claims priority from Israel PatentApplication No. 236544, filed Dec. 31, 2014.

FIELD OF THE INVENTION

The present invention relates to printed circuit boards (PCBs) andprinted circuit assemblies (PCAs) in general, and to the management ofthermal issues associated with a PCB or PCA, in particular.

BACKGROUND OF THE INVENTION

Printed circuit boards (PCBs) are used to mechanically support andelectrically connect different electronic components using conductivetracks, pads and other features etched from conductive sheets, typicallycopper. These copper sheets are typically laminated onto anon-conductive substrate. PCBs, also called printed wiring boards(PWBs), can be single sided (with one conductive layer), double sided(two conductive layers), or even multi-layer.

When choosing PCB substrate materials, the mechanical, electrical,chemical, and thermal properties of the material should be taken intoconsideration. A commonly used resin for commercial applications isFR-4, which is a designation given to a composite material of wovenfiberglass cloth with an epoxy resin binder that is flame resistant.Other di-functional and poly-functional epoxies may also be used. Theglass transition temperatures of these substrate materials (Tg)typically ranges from 125° C. to 170° C. Polyimide resins with a higherTg (>200° C.), long-term thermal resistance, and a lower coefficient ofthermal expansion (CTE) are used for high-performance multilayer PCBswith a large number of layers.

In the recent past, heat loads on printed circuit assemblies (PCAs) haveincreased significantly, in some cases rising from approximately 30 W to130 W. Standard VME (Versa Module Europa) conduction cooling designs(such as VITA 46/48.2) struggle to provide adequate heat dissipation forsuch a power level, and are strongly dependent on the amount of ECS(Environmental Control System) cooling available. Most of the VMEmodules consist of a host PCA and one or two mezzanine boards. In thepast, most of the heat was concentrated on the host side of the VMEmodule, while nowadays the mezzanine's power dissipation has increasedsignificantly due to their greater functionality. Designers use themezzanine boards to locate high power central processing units (CPUs) orgraphics processing units (GPUs), which can generate high powerdissipation of around 50 W. For fan cooling platforms, power limitationsare also restricted due to the increase of the components operationaltemperatures. These factors can cause a reduction in reliability andservice life of the boards.

Some cooling solutions for high power PCAs are described in the VITA48.3 Liquid Cooling standard. However, liquid cooling solutionssignificantly increase the dimensions of the cooling system since theyrequire additional items (such as: a liquid to air heat exchanger, apump, a volume compensator, and the like). Other cooling solutions forhigh power PCBs are described in the VITA 48.5 Air Flow Through Cooling(AFTC) standard. Such an AFTC configuration includes a single airflowchannel that extends through an interior cavity of a main chassis unit,as disclosed for example in U.S. Pat. No. 7,995,346 to Biemer et al. Asthe electronic modules are enclosed within the main chassis unit, theairflow provides cooling of the interior modules without directlyexposing the module electronics to direct contact with air, whicheliminates the risk of exposure to contaminants in the air.

While such an AFTC configuration suggests a solution to overcome theactual heat power requirements for many PCAs, there remain severallimitations which restrict board design, such as the location of highpower components, e.g., central processing units (CPUs), graphicsprocessing units (GPUs) and field-programmable gate arrays (FPGAs).Since the VITA 48.5 AFTC is characterized by a single airflow channelwith a single inlet and a single outlet, the location of the highestpower dissipation is constrained to be located near the inlet area. Thisrestriction can force the designer to add several high power componentsin the same board, which can result in poor thermal distribution on theboard. Furthermore, it can affect the design of the heat exchanger whichcan cause pressure drop. In order to overcome the heat dissipation, theheat exchanger density has to increase and therefore the pressure dropthrough the channel increases. The increase in pressure drop inducesseveral limitations on the cooling system, aircraft environmentalcontrol system (ECS), or fan cooling.

Conductors on different layers of the PCB are connected with holes inthe PCB, which are referred to as “vias”. The most common type of via isa “plated through hole” (PTH), which is formed by drilling a holethrough the multilayer PCB and electrochemically plating the hole with aconductive metal, usually copper, providing electrical connectionsbetween the layers. Vias may be formed in a variety of configurations. Acommon configuration is the “stub via”, in which the though portionextends from the top layer to an inner layer, while the stub portioncontinues from the inner layer junction to the bottom layer.Alternatively, a first stub extends from the top layer to a first innersignal layer, a through portion continues to a second inner layer, and asecond stub continues from the second inner layer to the bottom layer. A“through via” is a basic configuration in which there are no stubs butonly a through hole extending between the external layers. A “blind via”originates at an external layer and terminates at some inner layer,while a “buried via” connects one or more internal layers only (withoutconnecting to any external layer). A “back-drilled via” is formed usingthe post-fabrication back-drilling process to remove the stub portionsof a PTH via, used in relatively thick PCBs such as thick high-speedbackplane designs.

One well known failure mode associated with PCB reliability is thephenomenon of via cracking and PTH fatigue. Via cracking occurs due tothe out-of-plane/z-axis mismatch of the coefficient of thermal expansion(CTE) between the copper plating of a PTH via (approximately 17 ppm/°C.) and the surrounding substrate materials (approximately 45-70 ppm/°C.). This material property mismatch leads to differential expansionduring temperature variations and the formation of cracks in the viabarrel and inner layers as a result of mechanical fatigue. When exposedto thermal cycling, the initiated via cracks may propagate along thecopper plated barrel and gradually expand, leading to degradation andeventual PTH failure. In particular, via cracks continually affectelectrical discontinuities in the PCB, which can ultimately result incatastrophic failure of the entire PCB-based device. Such a failure isparticularly problematic in certain technical fields, such as variousmilitary, aerospace, automotive, and medical device applications. PTHfatigue is influenced by various parameters, such as: the maximum andminimum temperatures; the PTH diameter; the copper plating thickness andmaterial properties (e.g., ductility, yield strength); the substratethickness and material properties (e.g., CTE, elastic modulus); anddefects in the copper plating (e.g., voids, folds, etch pits). Referenceis made to FIG. 1, which is a cross-sectional schematic illustration ofa plated-through hole via with via cracks, as known in the art.

Numerous publications and industry joint efforts cover the keyparameters that affect thermal cycle reliability. The strainaccumulation depends on strain level induced at each thermal cycleexcursion. There are several known approaches for increasing the usefulPCB life cycle (i.e., number of thermal cycles before failure). Oneapproach is to decrease the PTH aspect ratio, defined as the ratiobetween the board thickness and the PTH diameter, for example to lessthan about 5:1. The smaller the aspect ratio, the more consistent theplating throughout the length of the via. Large aspect ratio vias tendto have greater plating thickness at each end as compared to the centerof the barrel, which increases the likelihood of cracked via barrels dueto z-axis expansion when soldering. However, decreasing the PTH aspectratio is extremely difficult (if not impossible) in the current era ofHigh Density Interconnect (HDI) PCB designs. Another approach is toincrease the copper (Cu) plating thickness, thereby increasing the spacefor the via crack to propagate. However, the thickness should beoptimized for the process to reduce stress risers due to defects (suchas voids in the Cu plating). A standard copper plating thickness isapproximately 25 μm, and increasing the thickness beyond around 35 μm isextremely difficult from a manufacturing standpoint.

A further approach would be to improve and test for the adhesion of theCu foil and the laminate. This option may be theoretically possible, butremains unproven. Yet another approach would be to utilize a Cu materialwith higher ductility and lower strength, although such a material hasnot yet been identified commercially. Other options involve decreasingor eliminating thermal shocks during board processing (such as bypreheating the board before hot-air leveling, wave soldering, rework,etc), or decreasing the range of thermal cycling, specifically avoidingexposure to temperatures above the glass-transition temperature (T_(g))of the resin. These options are not feasible since the board processingrequires several necessary thermal processes.

U.S. Pat. No. 8,427,828 to Kehret et al. describes a printed circuitboard module that includes a thermal shunt that provides a path betweenat least some of the electronic components and the front surface of theenclosure. U.S. Pat. No. 8,477,498 to Porecca et al. describes aconduction-cooled apparatus that includes conduction flow paths betweena circuit card and the enclosure system. U.S. Pat. No. 8,482,929 toSlaton et al. describes a system for circuit board heat transfer that isconfigured to transfer heat from the PCB to the chassis through athermal interface material and a thermal via. U.S. Pat. No. 7,459,200 toMcCall et al. describes a circuit board where the fiberglass fibers aredisposed in a two-dimensional pattern. U.S. Pat. No. 7,973,244 to Lin etal. describes a printed circuit board that includes a base formed from aplurality of woven fibers, and signal traces laid on the base.

U.S. Pat. No. 6,447,886 to Mohamed et al., entitled “Base material for aprinted circuit board formed from a three-dimensional woven fiberstructure”, discloses a printed circuit board constructed from a basematerial formed from a three-dimensional orthogonally woven fabrichaving a crimp-free fiber architecture in the x-y plane and anintegrated multi-layer structure. The base material includes: a firstsystem of straight first fibers extending along a first direction in afirst plane, a second system of straight second fibers extending along asecond direction in a second plane parallel to the first plane, and athird system of third fibers extending along a third direction throughthe first and second systems and binding the first and second fibers.The direction of the third fibers may be orthogonal to the respectivedirections of the first and second fibers. A filler material, such asresin, coats a portion of the first, second and third systems. Theprinted circuit board further includes one or more conducive layersattached to the surfaces of the base material.

U.S. Pat. No. 5,379,193 to Gall et al., entitled “Parallel processorstructure and package”, is directed to a parallel processor packagehaving a plurality of microprocessors and memory modules mounted onprinted circuit boards. The printed circuit boards are mounted on aplurality of circuitized flexible substrates or “flex strips”, whichconnect the separate boards through a relatively rigid central laminateportion. The central laminate portion provides XY plane and Z-axisinterconnection and communication (inter-processor, inter-memory,inter-processor/memory and processor to memory bussing). The planarcircuitization, as data lines, address lines, and control lines, are onthe individual printed circuit boards, which are connected through thecircuitized flex, and communicate with other layers of flex throughZ-axis circuitization (via and through holes) in the central laminateportion.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there is thusprovided a printed circuit board (PCB) with enhanced durability. The PCBincludes a plurality of PCB layers. At least one PCB layer includes abase material formed from a three-dimensional (3D) woven fiberglassfabric impregnated with a resin. The 3D woven fiberglass fabric includesa first group of fibers arranged in a plurality of parallel layers,where each layer includes a plurality of fibers extending along at leasta first (x-axis) direction and aligned in a first (x-y) plane, and wherethe parallel layers are arranged along a second (z-axis) direction thatis orthogonal to the first (x-y) plane. The 3D woven fiberglass fabricfurther includes a second group of fibers extending along at least thesecond (z-axis) direction, the second group of fibers being interlacedwith the first group of fibers. Each PCB layer further includes a layerof conductive material deposited onto a surface of the base material,and at least one conductive plated through hole (PTH) extending throughthe base material of multiple PCB layers. The coefficient of thermalexpansion (CTE) of the base material along the second (z-axis) directionsubstantially matches the CTE of the conductive material along the first(x-axis) direction. The second group of fibers may be interlaced withthe first group of fibers in a non-orthogonal weaving pattern, such asan angle interlock weaving pattern or a multilayer weaving pattern. Thefirst group of fibers or the second group of fibers may have a solidgeometry, a hollow geometry, a shell geometry, or a nodal geometry. The3D woven fiberglass fabric may include E-glass fibers or FR-4fiberglass.

In accordance with another aspect of the present invention, there isthus provided a method for fabricating a PCB with enhanced durability.The method includes the procedure of forming a base material for atleast one PCB layer from a three-dimensional (3D) woven fiberglassfabric impregnated with a resin. The 3D woven fiberglass fabric includesa first group of fibers arranged in a plurality of parallel layers,where each layer includes a plurality of fibers extending along at leasta first (x-axis) direction and aligned in a first (x-y) plane, and wherethe parallel layers are arranged along a second (z-axis) direction thatis orthogonal to the first (x-y) plane. The 3D woven fiberglass fabricfurther includes a second group of fibers extending along at least thesecond (z-axis) direction, the second group of fibers being interlacedwith the first group of fibers. The coefficient of thermal expansion(CTE) of the base material along the second (z-axis) directionsubstantially matches the CTE of the conductive material along the first(x-axis) direction. The method further includes the procedures ofdepositing a layer of conductive material onto a surface of the basematerial, and forming at least one conductive PTH extending through thebase material of multiple PCB layers. The second group of fibers may beinterlaced with the first group of fibers in a non-orthogonal weavingpattern, such as an angle interlock weaving pattern or a multilayerweaving pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with thedrawings in which:

FIG. 1 is a cross-sectional schematic illustration of a plated-throughhole via with via cracks, as known in the art;

FIG. 2 is a perspective view illustration of a PCA that includes an airflow through (AFT) cooling mechanism with multiple air flow channels,constructed and operative in accordance with an embodiment of thepresent invention;

FIG. 3 is a perspective view illustration of a PCA that includes an airflow through cooling mechanism with multiple channels havingperpendicular inlets and outlets, constructed and operative inaccordance with another embodiment of the present invention;

FIG. 4 is a schematic illustration of a PCB manufacturing process with a3D woven fabric for the non-conductive substrate material, operative inaccordance with an embodiment of the present invention;

FIG. 5A is a perspective view illustration of an exemplary configurationof a segment of 3D woven fabric with an orthogonal weaving pattern,constructed and operative in accordance with an embodiment of thepresent invention;

FIG. 5B is a cross-sectional illustration of a 3D woven fabric with anangle-interlock weaving pattern, constructed and operative in accordancewith another embodiment of the present invention;

FIG. 5C is a perspective view illustration of a 3D woven fabric with amultilayer weaving pattern, constructed and operative in accordance witha further embodiment of the present invention;

FIG. 6A is an exploded longitudinal view illustration, and an explodedcross-sectional view illustration of an orthogonal weave pattern fiberexhibiting drill smear, operative in accordance with an embodiment ofthe present invention;

FIG. 6B is an exploded longitudinal view illustration, and an explodedcross-sectional view illustration of an angular weave pattern fiberwithout drill smear, operative in accordance with another embodiment ofthe present invention; and

FIG. 7 is a chart depicting the simulation results of mean fatigue lifefor PCB materials with different parameters, using the IPC-TR-579Failure Model, operative in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention overcomes the disadvantages of the prior art byproviding thermal management approaches for a printed circuit board(PCB) and/or printed circuit assembly (PCA). The terms “printed circuitboard (PCB)” and “printed wiring board (PWB)” are considered analogousand are used interchangeably herein. Correspondingly, the terms “printedcircuit assembly (PCA)” and “printed wiring assembly (PWA)” areconsidered analogous and are used interchangeably herein, where a PCA isused to refer to the PCB board together with the electronic componentsassembled on the board. A first thermal management approach involves anair flow through cooling mechanism with multiple airflow channels forreducing the heat load in a PCA, where the air flow direction through atleast one of the channels is different from the air flow directionthrough at least another of the channels. Alternatively or additionally,the airflow inlet of at least one of the channels is off-axis withrespect to the airflow outlet of the channel. A second thermalmanagement approach involves a fabrication technique for a PCB involvinga 3D woven fabric, which helps mitigate the phenomenon of via crackingor PTH fatigue and increase the mean time between failures (MTBF) of thePCB.

According to one embodiment of the present invention, the multipleairflow channels of an individual PCA board are configured to directcooling air in different (e.g., opposite) directions. Reference is nowmade to FIG. 2, which is a perspective view illustration of a PCA,generally referenced 100, that includes an air flow through (AFT)cooling mechanism with multiple airflow channels, constructed andoperative in accordance with an embodiment of the present invention.Printed Circuit assembly (PCA) 100 includes a host-frame 102, ahost-board 104, a mezzanine-frame 106, and a mezzanine-board 108. Afirst surface (e.g., “top side”) of host-board 104 is mounted or securedto one side of host-frame 102, while an opposite surface (e.g., “bottomside”) of host-board 104 is mounted or secured to one side ofmezzanine-frame 106. Mezzanine-board 108 is mounted or secured to anopposite side of mezzanine-frame 106. PCA 100 may be enclosed in ahousing or chassis, and may include any number of frames and boards.

Host-frame 102 and mezzanine-frame 106 each include an interior cavityor a substantially hollow core. Host-frame 102 includes multiple airflowchannels 110, 112. In particular, a first edge surface 111 of host-frame102 includes an inlet 110A of airflow channel 110, and a second edgesurface 113 of host-frame 102 includes an outlet 110B of airflow channel110. An inlet 112A of airflow channel 112 is on the second edge surface113 of host-frame 102, and an outlet 112B of airflow channel 112 is onthe first edge surface 111 of host-frame 102. Each inlet 110A, 112A andoutlet 110B, 112B is an opening defined as at least one aperturedisposed on the respective edge surface (111, 113) of the host-frame102. Accordingly, airflow channel 110 extends through the volume cavitywithin host-frame 102, between inlet 110A on one end and outlet 110B onthe other end. Similarly, airflow channel 112 extends in the oppositedirection through the volume cavity within host-frame 102 between inlet112A and outlet 112B. Correspondingly, mezzanine-frame 106 includesairflow channels 114, 116. In particular, airflow channel 114 extendsthrough the interior cavity of mezzanine-frame 106 between inlet 114A ona first edge surface 115 and outlet 114B on a second edge surface 117 ofmezzanine-frame 106. Airflow channel 116 extends in the oppositedirection through the interior cavity of mezzanine-frame 106 betweeninlet 116A on second edge surface 117 and outlet 116B on first edgesurface 115.

Each air flow channel (110, 112, 114, 116) may include at least oneelement or mechanism configured to direct air flow along a selecteddirection along the respective airflow channels, such as internal fins.The fins (or other mechanism) may be configured in various ways, such asin accordance with the particular characteristics of the PCA and channeldesigns. It is noted that host-frame 102 and mezzanine-frame 106 maygenerally include any number of (multiple) airflow channels, while tworespective channels are depicted here for exemplary purposes only. It isfurther noted that PCA 100 may include multiple mezzanine-boards and/ormultiple mezzanine-frame, and further alternatively there may bemultiple boards mounted or secured within a single frame. For example,two additional mezzanine-boards may be mounted onto opposite sides ofhost-board 104 and attached to host-frame 102 to cool (resulting in atotal of four mezzanine-boards in the PCA).

Airflow channels 110, 112 serve to allow air to pass through, in orderto dissipate heat generated by the electronic components of host-board104 during its operation, particularly the electronic componentsdisposed on the regions of host-board 104 that are adjacent to airflowchannels 110, 112 within host-frame 102. In particular, air enters inlet110A and flows through channel 110 along the length of host-frame 102and exits from outlet 110B, thereby cooling the electronic components ofhost-board 102 situated near and along channel 110. Similarly, airenters inlet 112A and flows through channel 112 along the length ofhost-frame 102 and exits from outlet 112B (i.e., in an oppositedirection with respect to the air flow through channel 110), therebycooling the electronic components of host-board 102 situated near andalong channel 112.

Correspondingly, the airflow channels 114, 116 of mezzanine-frame 106serve to dissipate heat generated by the electronic components onmezzanine-board 108, as well as dissipating heat generated by theelectronic components on the host-board 104 which are situated adjacentto airflow channels 114, 116 of mezzanine-frame 106. In particular, airenters respective inlets 114A, 116A and flows through respectivechannels 114, 116 along the length of mezzanine-frame 106 and exits fromrespective outlets 114B, 116B, thereby cooling the electronic componentsof host-board 102 and mezzanine-board 108 situated near and alongchannels 114, 116.

PCA 100 is characterized by multidirectional airflow cooling, since afirst airflow channel outlet (110B) and a second airflow channel inlet(112B) are disposed on one edge surface (113) of host-frame 102, whilethe second airflow channel outlet (112A) and the first airflow channelinlet (110) are disposed on another edge surface (111) of host-frame102. As a result, the cooling air flows along channel 110 from inlet110A (edge surface 111) to outlet 110B (edge surface 113) in a firstdirection (i.e., from left to right as depicted in FIG. 2), whilecooling air flows along channel 112 from inlet 112A (edge surface 113)to outlet 112B (edge surface 111) in an opposite direction (i.e., fromright to left as depicted in FIG. 2). Such a multidirectional AFTchannel configuration may provide a more uniform temperaturedistribution and a reduction in the overall PWA temperature, as comparedto an AFT channel configuration with unidirectional airflow cooling.

According to another embodiment of the present invention, the airflowchannel outlet may be configured in an off-axis alignment relative tothe airflow channel inlet. Reference is now made to FIG. 3, which is aperspective view illustration of a PCA, generally referenced 120, thatincludes an air flow through cooling mechanism with multiple channelshaving perpendicular inlets and outlets, constructed and operative inaccordance with another embodiment of the present invention. PCA 120includes a host-frame 122 and a host-board 124. A surface of host-board124 is mounted or secured to one side of host-frame 122. Host-frame 122includes multiple airflow channels (132, 134, 136, 138, 140, 142)extending through an interior cavity of host-frame 122, where eachairflow channel is aligned in an “L-shaped” configuration. Inparticular, a first edge surface 126 (e.g., “top side”) of host-frame122 includes respective inlets 132A, 134A, 136A, 138A, 140A, 142A ofrespective airflow channels 132, 134, 136, 138, 140, 142. A second edgesurface 128 (e.g., “left side”) of host-frame 122, which issubstantially orthogonal to the first edge surface 126, includesrespective outlets 132B, 134B, 136B of respective airflow channels 132,134, 136. A third edge surface 130 (e.g., “right side”) of host-frame122, which is substantially orthogonal to the first edge surface 126,includes respective outlets 138B, 140B, 142B of respective airflowchannels 138, 140, 142. Accordingly, air passing through an airflowchannel 132, 134, 136, 138, 140, 142 changes direction, as it entershost-frame 122 from a respective inlet on edge surface 126 (i.e., fromthe top side) along a first axis, and then exits host-frame 122 from arespective outlet on edge surface 128 or 130 (i.e., from the left orright side) along a second axis that is different from the first axis,such that the air flows in an “L-shaped” pattern. As air passes throughthe airflow channels 132, 134, 136, 138, 140, 142 of host-frame 122, theheat generated by the electronic components of host-board 124 that aresituated adjacent to the respective channels is dissipated. Each airflow channel (132, 134, 136, 138, 140, 142) may include at least oneelement or mechanism (such as internal fins) configured to direct theair flow along selected directions through the respective airflowchannels. It is appreciated that the “off-axis inlet/outlet” airflowchannel configuration of PCA 120 may provide additional flexibility withregard to the layout of the board components, as compared to theparallel airflow channel configuration of PCA 100 (FIG. 2). Whereas asingle straight airflow channel (i.e., single axis air flow) would becharacterized by one cooled area and one uncooled (hot) area, anoff-axis inlet/outlet airflow channel configuration allows a designer todivide the board into two separate cooling areas and two separateuncooled areas, providing greater flexibility when designing the layoutof the integrated circuits on the board. For example, two high powercomponents may be positioned on each side of host-board 124 with thesame cooling conditions. Moreover, the off-axis inlet/outlet channelconfiguration of PCA 120 may provide a more uniform temperaturedistribution, as well as a substantially lower pressure drop for thesame flow rate.

At least one of the airflow channels of PCA 100, 120 may becharacterized by a single inlet and multiple outlets, or alternatively,multiple inlets and a single outlet. For example, referring to FIG. 3, asingle airflow channel may extend from inlet 132A through to outlets132B and 134B on edge surface 128. For another example, a single airflowchannel may extend from inlet 132A through to outlets 132B and 142B onopposite edge surfaces 128, 130, resulting in a “T-shaped” channelconfiguration.

It is appreciated that the multiple airflow channels of PCAs 100, 120(FIGS. 2 and 3) provide greater airflow cooling, in comparison to thatprovided by an AFT cooling configuration with a single airflow channel.The enhanced airflow cooling reduces the temperature distribution on thePCA board, which also allows for increasing the amount and/or theoperational power of the electronic components mounted on the PCA board.In addition, multiple airflow channels allows the thermal distributionon the board to be more uniform as a function of the airflow per eachinlet and the power distribution, than would be the case with a singlechannel with a single inlet. Moreover, the PCA designer is provided withgreater flexibility regarding the distribution of the electroniccomponents on the board, particularly the high power components. It isalso noted that the multiple airflow channels results in a reduction inpressure drop in each channel, in comparison to an AFT coolingconfiguration with a single channel. Furthermore, having multiplechannels reduces the heat load concentration on each individual channel,and therefore reduces the heat flux concentration on the individualchannel. Multiple channels also increase the heat transfer area of theair (or other fluid) flowing through the channel, and increases thenumber, size and/or operating power of board components that can be incontact with the channel.

According to a further embodiment of the present invention, at least oneairflow channel may be selectively regulated by modifying a property orcharacteristic of the channel, such as by adjusting the aperturediameter of the channel inlet and/or the channel outlet, adjusting thechannel volume, or by changing the airflow direction through thechannel. For example, with reference to PCA 100 (FIG. 2), a controller(not shown) is configured to adjust the diameter of an inlet (110A,112A) and/or an outlet (110B, 112B) of an airflow channel 110, 112, inaccordance with real-time requirements. For example, if the electroniccomponents on host-board 104 situated near or along airflow channel 112are characterized with greater power loads and/or produce more heat ascompared to the components situated near or along airflow channel 110,then the controller may reduce the aperture size of inlet 110A and/oroutlet 110B, or alternatively or additionally, may increase the aperturesize of inlet 112A and/or outlet 112B, such that greater airflow willoccur through channel 112 than through channel 110. For example, thecooling air may be diverted entirely to airflow channel 112. In anotherexample, the controller may change the airflow direction through anairflow channel, such as by directing cooling air to flow in an oppositedirection in airflow channel 110 (i.e., from the previous outlet 110Bthrough the previous inlet 110A), or by establishing off-axis airflowfrom a different inlet and/or a different outlet of channel 110. Thedirection of air flow through a channel may be modified when there is anincrease or decrease in the heat load on a particular area of thehost-board. The controller may be coupled to at least one sensor (notshown), and may determine how or whether to adjust the properties of anairflow channel 110, 112 based on real-time feedback related to PCA 100as obtained by the sensor. For example, the sensor is configured todetect at least one parameter of host-board 104 and/or mezzanine-board108, such as the temperature level or current load (using a thermalsensor or a current sensor). The controller obtains an indication of therelevant parameter(s) from the sensor (e.g., real-time thermal level orcurrent load of different board components), and then selectivelymodifies the properties of at least one airflow channel 110, 112 (e.g.,by reducing or increasing at least one inlet/outlet aperture diameter,or by changing a direction of air flow) in accordance with the detectedparameter(s). In general, the properties of the airflow channels thatextend through the host-frame or mezzanine-frame may be selectivelyadjusted based on the real-time cooling requirements of the PCA. Forexample, a PCA may include multiple boards associated with differentapplications that each have different heat loads, such that greater airflow is directed through the airflow channels situated adjacent to theboards with greater heat loads (and vice-versa). The airflow cooling mayalso be selectively controlled on the level of the entire PCA chassis orhousing, such as by dispersing air differentially between differentframe pairs or board pairs (e.g., via non-unidirectional airflows).

In accordance with a second thermal management approach of the presentinvention, the nonconductive substrate or base material of the PCB isformed from a 3D woven fabric, such as 3D woven fiberglass impregnatedwith an epoxy resin, in order to mitigate the phenomenon of via crackingand PTH fatigue, thereby enhancing the PCB durability. Reference is nowmade to FIG. 4, which is a schematic illustration of a PCB manufacturingprocess with a 3D woven fabric for the nonconductive substrate material,operative in accordance with an embodiment of the present invention. Theraw materials for the PCB manufacturing process include a glass fabric(e.g., fiberglass), an epoxy resin, and a copper foil, where the glassfabric is a 3D woven fabric, as discussed further hereinbelow. The 3Dwoven glass fabric is impregnated with the resin, following which thecopper foil is laid up on and pressed together with the impregnatedglass fabric. Subsequently, the pressed material is divided up intoindividual boards. It will be appreciated that manufacturing processshown in FIG. 4 is essentially a standard PCB manufacturing process,where a 3D woven glass fabric is used for the raw materials for thenon-conductive substrate instead of a 2D glass fabric.

Reference is now made to FIG. 5A, which is a perspective viewillustration of an exemplary configuration of a segment of a 3D wovenfabric with an orthogonal weaving pattern, generally referenced 200,constructed and operative in accordance with an embodiment of thepresent invention. 3D woven fabric 200 includes a first group of fibers,referenced 202 and 204, and a second group of fibers, referenced 206.First group of fibers 202, 204 are stacked in multiple layers. Eachfiber 202 extends (lengthwise) along the x-axis direction, and rows offibers 202 are arranged sequentially along the y-axis direction, suchthat each layer of fibers 202 is aligned (in sequential rows) in acommon plane (the x-y plane). Similarly, each fiber 204 extends(lengthwise) along the y-axis direction, and rows of fibers 204 arearranged sequentially along the x-axis direction, such that each layerof fibers 204 is also aligned (in sequential rows) in the same x-yplane. Layers of fibers 202, 204 are arranged or stacked along anorthogonal direction (z-axis) that is orthogonal to the x-y plane, thusforming a matrix of fibers that extend along the x-axis and y-axisdirection and arranged along the z-axis directions. Second group offibers 206 are interlaced with the first group of fibers 202 and 204.Second group of fibers 206 are arranged in the z-axis direction, suchthat the length of each fiber 206 substantially passes through orextends along at least the z-axis (i.e., but may also traverse thex-axis or y-axis), while being interlaced through the first group offibers 202, 204.

A 3D woven fabric may be classified according to various parameters,including: shedding mechanisms; weaving processes; weavingpattern/architecture; geometries/structures; and interlacements andfiber axis. One classification of 3D woven fabrics involves aconventional 2D weaving process designed to interlace two orthogonalsets of threads (“warp” and “weft”), which produces an interlaced 2Dfabric on a 2D weaving device. Another classification uses aconventional 2D weaving process designed to interlace two orthogonalsets of threads (warp and weft) with an additional set of yarnsfunctioning as binder warps or interlacer yarns in thethrough-the-thickness or z-axis direction. This is referred to as“multilayer weaving” and produces an interlaced 3D fabric constitutingtwo sets of yarns on a 2D weaving device. A further classification usesa conventional 2D weaving process with three sets of yarns (ground warp,pile warp, pile weft) to produce pile fabrics, known as “2.5D fabrics”.The fabric is manufactured by cutting a simple 3D weave consisting oftwo 2D fabrics connected by interwoven pile threads, to form a “hairy”fabric. These 2.5D fabrics are impregnated with epoxy resin in thestandard manner, laminated and cured in an autoclave. Yet anotherclassification involves a conventional 2D weaving process with threesets of yarns to produce a non-interlaced fabric with yarns in the warp,weft and through-the-thickness (z-axis) directions. Yet a furtherclassification uses a 3D weaving process to interlace three orthogonalsets of yarns, to produce an “orthogonal weaving” pattern. The weavingshed operates both row-wise and column-wise. This produces a fullyinterlaced 3D fabric where all three sets of orthogonal yarns interlace,using a specifically designed 3D weaving machine. A final classificationinvolves a non-woven, non-interlaced 3D fabric forming process designedto connect three orthogonal sets of yarns together with no interlacing(weaving), interloping (knitting), or intertwining (braiding). Thisfabric is held together by a special binding process.

In accordance with an embodiment of the present invention, the 3D wovenfabric is characterized by an angular (i.e., non-orthogonal) weavingpattern or architecture, such as the multilayer and angle interlockweaving patterns shown in FIGS. 5B and 5C. FIG. 5B, is a cross-sectionalillustration of a 3D woven fabric with an angle-interlock weavingpattern, generally referenced 210, constructed and operative inaccordance with another embodiment of the present invention. FIG. 5C isa perspective view illustration of a 3D woven fabric with a multilayerweaving pattern, generally referenced 220, constructed and operative inaccordance with a further embodiment of the present invention.Orthogonal weaving pattern 3D fabric 200 is characterized by straightyarns in all three principal directions, and is therefore useful fortextiles where non-crimping is desired. Angle-interlock weaving pattern3D fabric 210 is a 3D solid configuration in which the weft yarns arekept straight while the warp yarns are directed to pass through thefabric architecture diagonally with defined depth. Wadding yarns can beadded in the warp direction to achieve high tensile modulus andstrength. Multilayer weaving pattern 3D fabric 220 results in a fabriccomposed of two or more layers. Each layer may have its own weave, andthe layers can be stitched to form an integral 3D architecture. Themultilayer weaving process is designed to interlace two sets of yarns(warp and weft) with an additional set of yarns functioning as binderwarps or interlacer yarns in the through-the-thickness or z-direction.It is noted that the angle interlock weaving pattern 210 and multilayerweaving pattern 220 can be produced with conventional 2D weavingmachines, especially shuttle looms, whereas the orthogonal weavingpattern 200 requires a specially designed 3D weaving machine.

The fiber geometry (or “structure”) of a 3D woven fabric can be selectedfrom a variety of known geometric forms, such as: solid, hollow, shelland nodal. A solid geometry has a compound structure, with a regular ortapered geometric shape. A solid geometry is typically used withmultilayer and angle interlock architectures. A hollow geometry providesa shape with both even surfaces and uneven surfaces, andmulti-directional tunnels on different levels. A hollow geometry istypically used with a multilayer architecture. A 3D-hollow structure hasopenings in the fabric cross-section, and so a multilayer weavingpattern can be applied to both hollow structures with flat surfaces andto hollow structures with wavy surfaces. A shell geometry has a shapewith spherical shells and/or open box shells. A shell geometry istypically used with a multilayer or single layer architecture. A nodalgeometry has a shape with tubular nodes and/or solid nodes. A nodalgeometry is typically used with multilayer and angle interlockarchitectures.

It should be noted that in a PCB formed from a 3D woven fabric with anon-orthogonal weaving pattern (such as angle-interlock or multilayer)and a solid fiber geometry, the phenomenon of “drill smear” whendrilling the PTH via is substantially minimized, as compared to a PCBcomposed of a 3D woven fabric with an orthogonal weaving pattern. Drillsmear relates to the formation of specks and debris caused during thedrilling process, which are difficult to remove and may cover theconductors in the inner layers and impair the PCB conductivity.Reference is made to FIGS. 6A and 6B. FIG. 6A is an explodedlongitudinal view illustration, referenced 232, and an explodedcross-sectional view illustration, referenced 234, of an orthogonalweave pattern fiber exhibiting drill smear, operative in accordance withan embodiment of the disclosed technique. FIG. 6B is an explodedlongitudinal view illustration, referenced 236, and an explodedcross-sectional view illustration, referenced 238, of an angular weavepattern fiber without drill smear, operative in accordance with anotherembodiment of the disclosed technique. An orthogonal weave patternresults in drill smear (as seen in illustrations 232 and 234), since thecross cut section of any single fiber is a rectangle along the holebarrel of the PTH 242, and therefore continuity of the copper plating244, is detrimentally impacted. In contrast, there is substantially nodrill smear when implementing an angle weave pattern (as seen inillustrations 236 and 238), since the cross cut section of any singlefiber is a semi-elliptical shape along the PTH 246. Thus, the continuityof the copper plating 248 in a 3D woven fabric with an angular(non-orthogonal) weave pattern is virtually similar to that of a 2Dfabric, in which the corresponding area is essentially angular.

The properties of the z-axis fibers 206, such as the type of weavingpattern and the angle at which the (z-axis) fibers 206 are interlacedthrough the (x-y plane) fibers 202, 204, may be selected in accordancewith the particular parameters and/or requirements of the PCB. Forexample, the z-axis interlacing angle is substantially acute, such asslightly less than 90 degrees (e.g., between 75-89 degrees), in order toavoid defects in the drilled hole and defects in the PTH copper plating(e.g., non-uniformity, dimpling, blistering, etc) that may result whendrilling along a singular fiber or cluster. In general, exemplaryparameters used for determining the properties of the z-axis fibers 206include: the number of layers; the fabric density; the aspect ratio; thetype of resin; and/or other relevant drilling parameters for producing adefect free hole after the copper plating.

Referring back to FIGS. 4, 5A, 5B and 5C, a layer of conductive materialis deposited or disposed onto at least one surface of the non-conductivesubstrate of a PCB, such that one or more conductive PTH vias extendthrough the substrate to connect different layers of the PCB. 3D wovenfabric (200, 210, 220) includes fibers 206 that are arranged in thethrough-the-thickness (z-axis) direction. As a result, the coefficientof thermal expansion along the z-axis direction (z-CTE) of a PCBsubstrate composed of a 3D woven fabric (200, 210, 220) substantiallymatches the z-CTE of the PCB via copper plating (e.g., approximately 17ppm), thereby decreasing the thermal expansion of the PCB substrate overthermal cycling. In this manner, the phenomenon of via cracking (PTHfatigue) is substantially alleviated, and the ratio of thermal cycles tofailure, or the “mean time between failures (MTBF”), is substantiallyincreased (i.e., in comparison to a standard PCB with a substratemanufactured from a 2D woven fabric). Essentially, a standard PCB havinga substrate composed of 2D woven fabric is anisotropic (i.e., thematerial properties depend on the direction), at least with respect tothe CTE properties (i.e., the z-CTE is substantially different than thex-CTE and y-CTE). In contrast, a PCB with a 3D woven fabric basedsubstrate is isotropic (i.e., the material properties are identical inall direction) in terms of the CTE, or perhaps “quasi-isotropic” in thecase of the angular or non-orthogonal 3D fabric weaving patterns, suchthat the z-CTE substantially matches the x-CTE and y-CTE of thematerial. For comparison purposes, a typical x-CTE and y-CTE forcommercially available 2D woven fabric is approximately 17 ppm below theglass transition temperature (Tg), while a typical z-CTE isapproximately 50 ppm below Tg. The term “isotropic” is used herein in asufficiently broad manner to encompass scenarios in which materialproperties are “substantially similar” in all directions (but notnecessarily “identical”), for example encompassing the “quasi-isotropic”configuration of angular weaving patterns 210, 220.

In addition, if the 3D woven fabric is characterized with anon-orthogonal weaving pattern (such as patterns 210, 220), the z-axisfibers 206 of the 3D woven fabric (210, 220) are positioned at different(non-uniform) angles. Accordingly, the PCB substrate fabricated with thenon-orthogonal pattern 3D woven fabric (210, 220) is characterized byreduced non-uniformity at the interface between the conductive surfaceand the nonconductive substrate when the PTHs are drilled, which furtherserves to substantially minimize defects in the drilled hole and defectsin the PTH copper plating, such as non-uniformity, dimpling, blistering,drill smear, and the like.

3D woven fabric 200, 210, 220 may be fiberglass, such as E-glass fibersor FR-4 fiberglass. The conductive material of the PCB may be, forexample, copper (Cu), such that the PTH that extends through the PCBsubstrate is copper-plated. The PCB may include any type of via with anyconfiguration, including but not limited to: a stub via, a through via,a blind via, a buried via, and/or a back-drilled via. The thickness ofthe fibers may be between approximately 10-20 μm. 3D woven fabric 200,210, 220 may optionally include a surface treatment, such as primer orplasma, in order to improve the flow of the resin and its final adhesionto the fabric surface during the impregnation step of the PCBmanufacturing process (FIG. 4).

Reference is now made to FIG. 7, which is a chart, generally referenced250, depicting the simulation results of mean fatigue life (MTF) for PCBmaterials with different parameters, using the IPC-TR-579 Failure Model,operative in accordance with an embodiment of the present invention. Themean fatigue life is defined as the number of thermal cycles beforefailure, where “failure” is defined as a percentage increase inresistance of the PCB above the initial resistance measurement at agiven test temperature. The simulation calculations are based onparameters such as: the type and thickness of the PCB material, the PTHsize, and the expected thermal environment. In chart 250, allsimulations were based on a generic PCB material with a thickness of 3mm and a PCB elastic modulus of 3447 MPa, but varying values of z-CTE(50 ppm/° C.; 33 ppm/° C.; 17 ppm/° C.). All simulations of chart 250were based on a PTH diameter of 0.3 mm, and PTH wall thickness of 0.025mm, but varying values of PTH quality factors (either “poor” or “good”,as defined under the IPC-TR-579 Failure Model standard in relation tothe workmanship of a shop or supplier). All simulations of chart 250were based on a minimum temperature of −30° C. and a maximum temperatureof 71° C. From the simulation results, it is evident that a PCB materialwith a z-CTE of 17 ppm/° C., such as a PCB composed from a 3D wovenfabric in accordance with the present invention, results in asignificantly larger “cycles to failure” value as compared to the otheroptions. More particularly, the resultant cycles to failure value of a3D fabric reinforced PCB is “virtually infinity”, and thus thedurability improvement of such a PCB can be considered as conceivablyunlimited, with respect to other PCBs that are characterized byanisotropic CTE properties. It is further noted that whereas a “goodquality” PCB improves the cycles to failure ratio by at least an orderof magnitude for the two non-3D fabric based PCBs (i.e., the “standard”and “best” solutions), the cycles to failure of the 3D fabric reinforcedPCB remains “virtually infinity” regardless of whether the PCB qualityis “poor” or “good”. Thus, even a poor quality material provides anessentially unlimited improvement in PCB durability and mean fatiguelife for a 3D woven fabric based PCB.

It will be appreciated that a variety of methods for providing thermalmanagement for a PCB and/or PCA result from the above description. Onesuch method for providing thermal management is a method for cooling aPCA that includes a host-frame and at least one host-board mounted tothe host-frame. The method includes the procedure of dissipating heatgenerated by the host-board using a plurality of airflow channelsextending through respective apertures defined through a volume cavitywithin the host-frame, where each of the channels includes at least oneinlet at a first edge surface of the host-frame, and at least one outletat a second edge surface of the host-frame. At least one of the airflowchannels is configured to direct air flow in a first direction, and atleast another one of the airflow channels is configured to direct airflow in a second direction that is different from the first direction.Alternatively or additionally, at least one of the airflow channels isconfigured with at least one inlet that is off-axis with respect to atleast one outlet thereof.

Another such method for providing thermal management is a method forfabricating a PCB with enhanced durability. The method includes theprocedure of forming a base material from a 3D woven fiberglass fabricimpregnated with a resin. The 3D woven fiberglass fabric including afirst group of fibers arranged in a plurality of parallel layers, whereeach layer includes a plurality of fibers extending along at least afirst (x-axis) direction and aligned in a first (x-y) plane, and wherethe parallel layers are arranged along a second (z-axis) direction thatis orthogonal to the first (x-y) plane. The 3D woven fiberglass fabricfurther includes a second group of fibers extending along at least thesecond (z-axis) direction, the second group of fibers being interlacedwith the first group of fibers, such as in a non-orthogonal weavingpattern. The CTE of the base material along the second (z-axis)direction substantially matches the CTE of the conductive material alongthe first (x-axis) direction. The method further includes the proceduresof depositing a layer of conductive material onto a surface of the basematerial, and forming at least one conductive PTH extending through thebase material of multiple PCB layers.

It will be appreciated that the different embodiments of the presentinvention are not mutually exclusive and can be combined in variouscombinations to form a single embodiment including one or more of thedifferent aspects (e.g., multi-directional airflow channels, multipleairflow channels with off-axis inlets and outlets, and PCB substratecomposed of a 3D woven fabric). For example, the respective boards 120,122 of PCAs 100 or 120 (characterized by multi-directional airflowcooling channels and/or off-axis airflow channel inlets/outlets) can befabricated with a 3D woven fabric as in PCBs 200, 210, 220. Furthermore,the present invention can be combined or used in conjunction with othertypes of known internal or external cooling sources or heat mitigationdevices, such as fans or heat sinks.

While certain embodiments of the disclosed subject matter have beendescribed, so as to enable one of skill in the art to practice thepresent invention, the preceding description is intended to be exemplaryonly. It should not be used to limit the scope of the disclosed subjectmatter, which should be determined by reference to the following claims.

The invention claimed is:
 1. A printed circuit assembly (PCA)comprising: at least one host-board, comprising electronic componentsmounted thereon; and a host-frame, said host-board mounted to saidhost-frame, said host-frame comprising a plurality of airflow channelsextending through respective enclosed conduits defined through a volumecavity within said host-frame, each of said airflow channels comprisingat least one inlet at a first edge surface of said host-frame, and atleast one outlet at a second edge surface of said host-frame, each ofsaid airflow channels configured to pass through respective cooling-airthrough a respective enclosed conduit separated from other airflowchannels, while isolating the electronic components from saidcooling-air, wherein said airflow channels comprises: a first airflowchannel configured to direct a first cooling air flow at a firsttemperature and in a first direction through a first enclosed conduit,and a second airflow channel configured to direct a second cooling airflow at a second temperature through a second enclosed conduit in asecond direction different from said first direction, to selectivelycool the electronic components adjacent to each of the respectiveenclosed conduits at respective temperatures according to respectivecooling requirements, wherein the second enclosed conduit is not linkedwith the first enclosed conduit such that the second cooling-air doesnot interact with the first cooling-air.
 2. The PCA of claim 1, whereinfor at least one of said airflow channels, said outlet at said secondedge surface is orthogonal to said inlet at said first edge surface. 3.The PCA of claim 1, comprising a first airflow channel comprising aninlet at said first edge surface of said host-frame and an outlet atsaid second edge surface of said host- frame, and further comprising asecond airflow channel comprising an inlet at said second edge surfaceof said host-frame and an outlet at said first edge surface of saidhost-frame.
 4. The PCA of claim 1, further comprising a controller,configured to selectively adjust at least one channel property of atleast one of said airflow channels, in accordance with at least onedetected parameter of said host-board.
 5. The PCA of claim 4, whereinsaid channel property is selected from the list consisting of: anaperture diameter of the channel inlet or channel outlet; the volumecavity of the airflow channel; and at least one direction of air flowthrough said channel.
 6. The PCA of claim 4, wherein said detectedparameter is selected from the list consisting of: temperature level;and current load.
 7. The PCA of claim 4, further comprising at least onesensor coupled with said controller, said sensor selected from the listconsisting of: a thermal sensor; and a current sensor.
 8. A method forcooling the PCA of claim 1, the method comprising: passing throughrespective cooling-air in each of the airflow channels through therespective enclosed conduit separated from other airflow channels, whileisolating the electronic components from the cooling-air, to selectivelycool the electronic components adjacent to each of the respectiveenclosed conduits at respective temperatures according to respectivecooling requirements.
 9. The method of claim 8, further comprisingselectively adjusting at least one channel property of at least one ofthe airflow channels, in accordance with at least one detected parameterof the host-board.
 10. The method of claim 8, wherein at least oneairflow channel is configured with at least one inlet that is off-axiswith respect to at least one outlet thereof.
 11. The PCA of claim 1,wherein at least one airflow channel is configured with at least oneinlet that is off-axis with respect to at least one outlet thereof.